The Klingon name for the extensive redundancy built into their anatomy, including twenty-three ribs, two livers and an eight-chambered heart, which lets a warrior survive wounds that would kill most humanoids.
Brak'lul is the Klingon word for a striking feature of their physiology: almost every vital function has a back-up. A Klingon has twenty-three ribs, two livers, an eight-chambered heart and duplicated neural pathways, so that the loss of one organ in battle need not be fatal.
The term is introduced in the Next Generation episode Ethics, after a falling cargo container crushes Worf's spine. The visiting neurogeneticist Doctor Toby Russell regards the redundancy as an untidy over-design, while Doctor Crusher points out that it is exactly what keeps Klingons alive.
She is proved right at the end of the episode. Worf apparently dies on the operating table during Russell's experimental genetronic procedure, but a redundant set of synaptic functions takes over and he revives, to the astonishment of both doctors and the relief of his son Alexander.